Magnetic Contact Switches

A simple but useful type of sensor to detect the open/closed state of a door or window is a magnetic contact switch shown in the figure below. It consists of a magnet and reed switch in separate enclosures (ABS plastic or metal), where the switch turns on/off when the magnet is near it.

This overview of magnetic contact switches covers its usage and switch configurations.

Usage

The magnetic contact switch is typically used by mounting the magnet on the door (or moving part) and the switch on the door frame (stationary part) as shown in the figure below.

Switch Configurations

The magnetic contact switch is available in three different configurations:

Normally Closed (NCNormally Closed (NC) switch configuration. In its normal default state, the switch allows current to flow through the circuit. When activated, the switch opens, breaking the circuit and interrupting current flow.)
Normally Open (NONormally Open (NO) switch configuration. In its normal default state, the switch prevents current from flowing through the circuit (open circuit). When activated, the switch closes, allowing current to flow through the circuit.)
Normally Closed or Normally Open (NC/NONormally Closed or Normally Open (NC/NO) switch configuration. These switches have both NC and NO options where you can choose which configuration you want for your application.)

Each configuration is described below with figures illustrating inside the enclosures of both the switch and magnet.

Normally Closed (NC)

A NC magnetic contact switch means that the switch is normally closed without the magnet and conducts electricity (i.e., the door opening and removing the magnet would close the switch and allow the current to flow) and the switch opens when the magnet is near it, not allowing current to flow.

The benefit of this configuration is its not consuming power while the door is closed. One drawback is it may be difficult to determine if or when the sensor becomes accidentally disconnected until the door is opened.

Normally Open (NO)

A NO magnetic contact switch means that the switch is normally open without the magnet and cannot conduct electricity (i.e., the door opening and removing the magnet would open the switch, not allowing current to flow) and the switch closes when the magnet is near it to allow current to flow.

Magnetic Contact Switch Normally Open (NO) with Magnet Near

The benefit of this configuration its easy to determine if or when the sensor is accidentally disconnected when the door is closed by showing the wrong state. One drawback is it will be constantly consuming power while the door is closed.

Normally Closed or Open (NC/NO)

These magnetic contact switches have both NC and NO options with three terminals: NC, NO, and COMCommon Ground, where COM is the common ground. This has the advantage of letting you choose which configuration you want for your application.

Magnetic Contact Switch Normally Closed or Open(NC/NO) with Magnet Near

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Products

There is a common misunderstanding among vendors selling these switches that think the "normal" position is with a magnet next to the reed switch, probably because the normal position of a door/window is usually closed. Sometimes you see the NC or NO in the title of the product is often backwards. Before purchasing, it is recommended that you check for any clarification in the product description, product reviews, or Q&A comments.